UK : Amazon.co.uk | Blackwells | The Internet Bookshop | The Book Depository (offers free delivery worldwide on all books)
North America : Amazon.com | Barnes and Noble
France : Chapitre.com | Amazon.fr | Fnac | Bouquinerie L'Ex-Libris | La Procure (for religious books, especially)
Germany : Amazon.de
Australia : Bookworm | Dymocks
Korea : Kyobo (direct link to Search), Yongpung, Yes24, AladdinUS (These sites are all in Korean but AladdinUS also has an English-language page and services based in the US).
To buy books, videos and DVDs etc related to Korea in English (or with English subtitles) the Seoul Selection online bookstore is a major resource. They welcome information about new publications. They also publish a number of titles themselves. Books about Korea can also be purchased from Koryo Books located in the US. If you are in Seoul, Seoul Selection is also a place you can visit in person to browse or drink coffee in since their bookstore is also a cafe.
For Korean software (and indeed software in any language) see World Language Resources' Korean list.
To buy foreign books inside Korea, Interpark
is recommended, though I have never used it.
Bookfinder.com is a very good resource for finding out -of -print books, but perhaps Alibris is better...
allows you to search the stocks of many independent bookellers for
and rare books.
Antique bookstores in Korea: Old
Books and Hwabong